Subsurface exploration



Jan- 25,1955 E. F. EGAN ET AL SUBSURFACE EXPLORATION 2 Sheets-Sheet 1Filed May 24, 1954 Jan. 25, 1955 E. F. EGAN ET AL SUBSURFACE EXPLORATION2 Sheets-Sheet 2 Filed May 24, 1954 United States Patent SUBSURFACEEXPLORATION Edmond F. Egan and Gerhard Herzog, Houston, Tex., assignorsto The Texas Company, New York, N. Y., a corporation of DelawareApplication May 24, 1954, Serial No. 431,888

17 Claims. (Cl. 250-435) This invention relates to a method of studyingsubsurface earth formations and more particularly to a method ofmeasuring the permeability of earth formations traversed by a well orbore hole. The principal purpose of the invention is the provision of amethod of this type by means for which accurate measurements can be madeand without the use of complicated equipment. This is acontinuation-in-part of our application Ser. No. 349,784, filed April20, 1953, now abandoned for Subsurface Exploration.

One of the objects of the invention is the provision of a method for usein water injection wells in which it is desired to know the amount ofwater passing into various increments of an exposed, more or lesspermeable formation. This is sometimes referred to as an injectivityprofile or permeability log of a well. It is also contemplated that themethod which will be described can be used for locating casing leaks ina well. For instance, it is frequently necessary or at least desirableto determine positively that the injected water is not leaking aroundthe casing shoe and disappearing into a thief zone or formation abovethe bottom of the casing.

Another object of the invention is to determine the preslence of andlocation of zones of lost circulation in a wel Still another use of theinvention is in connection with selective acidization. Thus, the problemfrequently arises of acidizing an oil section located under a gassection in such a way that the acid will be directed substantiallyentirely into the oil section rather than into the gas section. Again, asimilar procedure can be used to acidize an oil section of a formationwhere the oil slection is located directly above a water-containingsection.

The importance of secondary recovery or repressuring in oil fields hasincreased greatly during the last few years, and there is consequently areal need for a better understanding of the engineering aspects of theseprojects. Detailed studies of reservoir performance under waterfloodingconditions can be undertaken only when certain basic data are available,and among the data which is necessary is information concerning thevertical distribution of the injected water in a water injection well.While efforts have been made to obtain data of this kind, no realsuccess has been obtained prior to the development of the method whichwill be described.

In accordance with this invention as it is applied to the making of aninjection profile of a formation into which water is to be injected, twostreams of fluid such as water are pumped into the well, one streamthrough a string of tubing extending downwardly below the formation andthe other stream downwardly through the annular spaces between thetubing and the casing or the walls of the hole. The streams are pumpedsimultaneously but separately, and each stream is carefully metered atthe surface. The water pumped down through the tubing wlll. of course,flow upwardly around the tubing until it meets the water pumpeddownwardly around the tubing and an interface will exist between the twostreams or bodies of water. A small amount of a tracer material such asa radioactive substance is added to the water being pumped down theannular space around the tubing so that all of this water will beradioactive, while the water pumped down through the tubing will benon-radioactive. In order to locate the interface, a radioactivitydetector is passed through the tubing, its depth being recordedcontinuously, and from the record of the output of the detector thedepth of the interface can be ascertained, since Patented Jan. 25, 1955the response of the detector will change more or less suddenly when thedetector passes from the radioactive water into the non-radioactivewater, or vice versa.

The rates of injection or pumping of the two streams can be varied bymeans fo the pumps at the surface, the rates being adjusted so that atall times the sum of the rates remains constant. By increasing the ratioof the amount to the radioactive water pumped to the amount ofnon-radioactive water pumped, the interface will be forced downwardlythrough the well past the exposed walls of the formation or zone to beexamined. The rates of injection of the two streams are varied byincrements, and the interface will, therefore, move downwardly by steps,the vertical length of these steps depending upon the permeability ofthe formation. After each adjustment or change in rates of injection,the radiation detector is passed through the well and a record made ofthe depth of the interface after such adjustment. In this manner aninjectivity profile is made of the formation to be examined, and thisrecord will show clearly variations in the permeability of all of thesections or portions of the formation. As will be explained hereinafter,a generally similar procedure can be used for locating leaks behind thecasing, zones of lost circulation, the depth at which aciclizing of theformations will be most beneficial, etc.

For a better understanding of the invention, reference is made to theaccompanying drawing in which:

Fig. 1 is a vertical, sectional elevation through a well showing theapparatus necessary for making a water injection profile;

Figs. 2, 3, 4, and 5 are vertical sections through a well showingdiagrammatically the downward progress of the interface between theradioactive and the non-radioactive liquids and the radioactivity logsobtained for four positions of the interface;

Fig. 6 is a vertical sectional elevation through a portion of the wellin which it is desired to determine whether or not water is beinginjected into a desired formation below the casing or whether it isleaking upwardly around the casing into a thief zone; and

Fig. 7 is another vertical, sectional elevation through a portion of awell in which it is desired to force acid into part but not all of asingle formation.

Referring to the drawing, a well or bore hole 10 is shown as traversingseveral subsurface formations, including the formation 12, for which itis desired to make a water injection profile. The upper portion of thewell is shown as being provided with a casing 14 having a closed casinghead 16. A string of tubing 18 passes through the casing head 16 anddownwardly through the well to a point below the formation 12. At thesurface a pump 20 is connected to the casing head through a meter 22 andis adapted to pump a stream of water 24 downwardly into the well throughthe space between the casing 14 and the tubing 18. A small amount ofradioactive material such, for example, as radioactive iodine in theform of sodium iodide is added to the water 24 by means not shown,preferably before the water is taken into the pump 20. Another pump 26is shown as connected through meter 28 to the upper end of the tubing 18and is adapted to pump non-radioactive water 30 downwardly through thetubing. The non-radioactive water passes out of the bottom end of thetubing and upwardly around the tubing until it meets the radioactivewater 24 at the interface 32. It will be seen that if the pumps 20 and26 are adjusted to change their rates of pumping while the total amountof water pumped by both pumps remains constant, the interface 32 will becaused to move up or down in the hole, depending upon the two pumpingrates.

Shown as suspended within the tubing 18 is a radioactivity logginginstrument 34 containing a detector of gamma rays, the output of whichis conducted upwardly through the suspending cable 36. This cable passesover a suitable cable-measuring device 38 which continuously indicatesthe depth of instrument 34 in the hole, and

- then to a suitable amplifier 40 and a recorder 42. When of thedetector is made continuously by a recorder 42, and this is correlatedwith the depth of the detector in the hole as measured by the device 38.Thus, by passing the detector 34 through the hole and comparing pointsin the record at which the detector passes from the radioactive fluidwith the depth in the hole at which those points are registered, anaccurate measurement is made of the depth of the interface 32.

Referring now to Figs. 2 through 5 of the drawing, it will be seen thatas the ratio of the injection rates of the radioactive fluid 24 and thenon-radioactive fluid 30 are changed, the interface 32 between thefluids will move along the formation to be examined, as has beenexplained above. The sum of the rates of injection of radioactive water24 and non-radioactive water 30 is held constant and for purposes ofillustration we will assume that the two streams are being pumped intothe well at the total rate of gallons per minute. The rates of the twostreams delivered by the pumps 20 and 26 are first adjusted so thatnon-radioactive water is injected in the tubing at the total injectionrate of 20 gallons per minute. After a short period of time, anyradioactive water opposite the permeable section 12, indicated in Figs.2-5 as containing the cavity 44, will be replaced by the non-radioactivewater 30 being injected through the tubing. A radioactive log is thenrun or, in other words, the radiation detector 34 is passed downwardlythrough the tubing 18. This situation is illustrated in Fig. 2, and itwill be observed that the interface 32 is located at substantially thetop of the permeable formation. At this time, of course, none of theradioactive fluid 24 is entering the formation since the interface isstill at. the top of the cavity. As the ratio of radioactive water 24 tonon-radioactive water 30 is increased, the interface will move downexposing more of the permeable zone 12 to the injection of radioactivewater. The ratio of radioactive water to non-radioactive water ischanged by increments, and the location of the interface is found at theend of each increment by passing the radiation detector through thewell, as described above.

Fig. 3 illustrates this movement of the interface by changing theinjection rates to 5 gallons per minute of radioactive water and 15gallons per minute of nonradioactive water. The zone located between theinterfaces, or rather the two positions of the interface 32 shown inFigs. 2 and 3 and indicated by the bracket A in Fig. 3, is taking waterat the rate of 5 gallons per minute, or 25 per cent of the totalinjected water. Fig. 4 illustrates the additional movement of theinterface by changing the injection rates to 9 gallons per minute ofradioactive water in the casing and 11 gallons per minute ofnon-radioactive water in the tubing. This is indicated roughly by thebracket B in Fig. 4. The zone B is, therefore, taking water at the rateof 4 gallons per minute (9 G. P. M.-5 G. P. M.), or 20 per cent of thetotal injected water. Fig. 5 illustrates the position of the interfacewith injection rates of 19 gallons per minute of radioactive water inthe casing and one gallon per minute of non-radioactive water in thetubing.

Referring again to Fig. 1, it will be understood that the zone,orformation 12 illustrated in that figure, corresponds to the zoneindicated by the bracket 12 m Fi 2.

The method which has been described so far has several advantages overother injectivity profiling systems. For example, the detectinginstrument 34 is run in the tubing 18, and this results in anundisturbed interface which, of course, is highly desirable. There isalso less danger of the instrument becoming stuck than if it were runthrough the space between the tubing and the walls of the hole. Again,with the method which has been described, the two injection waters canbe, and preferably are, chemically identical since the small amount ofradioactive material will not change the chemical behavior of the water24. This is an important consideration, particularly if the formationcontains clay which may swell when contacted by fresh water. The twofluids being injected are completely miscible so there will be norelative permeability effects to take into consideration.

The system which has been described can be used in open hole, as isshown in the drawings, or m a cased hole where the casing has'beenperforated. Again, with this method, it is not necessary to take intoaccount variations in the bore hole diameter, such as is required insome of the other systems.- While in the method which has been describedthe ratios of the injected water have been described as being varied asmuch as 5 gallons per minute at a time, it is to be understood that verysmall depth increments can be used, inasmuch as the lower limit of theincrements is determined only by the resolving power of the surfacepumps and meters. If desired, with one survey of the type which has beendescribed, one may obtain an injectivity profile of the zone beingflooded, check the casing for leaks, and evalu ate almost any kind ofremedial work which has been performed on the well.

Radioactive iodine in the form of sodium iodide has been found to be avery satisfactory tracer material. Due to the extensive use of thiscompound by the medical profession, its cost has been reduced to a pointwhere it is relatively inexpensive. Again, the half life of 8 days makesit very satisfactory from the health-hazard viewpoint. One of the mostdesirable features of this material is the fact that the non-metalliciodine, which is the radioactive atom of the compound, has no tendencyto replace any metallic ion which might be present in shaley material onthe well bore face. This feature enables the operator to make repeatruns since the radioactivity can be removed from the well bore simply byintroducing non-radioactive water.

While two streams of water, or at least a liquid, has been described asthe material to be injected into the formation, the permeability ofwhich it is desired to ascertain, it is also contemplated that gas canbe used. There are, of course, many wells in which it is desired toinject gas into one or more formations for the purposes of repressuring,or the like. Iodine can be dissolved in methane gas and elemental iodine131 is available, as are several iodine 131 compounds. From economic,half life, chemical, and ease in handling considerations, iodine wouldbe a very satisfactory tracer material to be used in the gas which wouldbe pumped down through the annular space between the tubing and casing,or the walls of the hole.

As was stated hereinbefore, the method substantially as it has beendescribed in connection with injectivity profiling can also be used forother purposes, for example, locating casing leaks. Fig. 6 of thedrawing illustrates a section of open hole 50 above which a casing 52has been set and cemented as indicated at 54. The lower portion of thehole passes through a permeable formation or zone 56 which it is desiredto use for injectivity purposes. It is not infrequent that when thecasing is cemented, voids such as indicated at 58 may occur in thecement, thus permitting fluid to leak upwardly around the cement anddisappear into another permeable formation 60 which is sometimesreferred to as a thief zone. The presence of such a leak can beascertained by means of the method which has been described. Thus, itmay be found that when the rates of injection are changed so as toincrease the rate of injection of the radioactive fluid 24 between thetubing and the casing as compared to the rate of injecting thenon-radioactive fluid 30 through the tubing 18, the interface 32 willnot move downwardly toward the formation 56, but will remain just belowthe lower end of the casing as is shown in Fig. 6. The position of theinterface will be located, of course, as has been described, by passinga radiation detecting instrument 34 through the string of tubing. Thefact that the interface 32 remains just below the bottom of the casingindicates, of course, that the radioactive water 24 is leaking upwardlypast the cement into the thief zone 60. Remedial steps can be taken tocorrect this situation, such as by recementing the casing.

If it should be found, as has been described above, that fluid isleaking around the bottom of the casing into a thief zone or zones, itmay be desired to locate the position of the thief zone so that stepscan be taken to seal it off as by cementing or by the use of some othersuitable plugging material. The radioactive liquid in flowing upwardthrough void 58 in the cement and then into one or more thief zones 60will, of course, produce an artificial radioactivity in these zones. Bymoving the radiation detecting instrument 34 through the lower portionof the casing, either during or after the time the radioactive liquid isflowing into the zone, the depth and thicknesses of the thief zone, orzone 60, can be readily ascertained since the detecting instrument willprovide an increased response when horizontally opposite a thief zone.After the zone has been located, steps can be taken as mentioned above,to seal off the zone. Thus, the casing 52 can be perforated eitheropposite or below the thief zone and cement or other plugging materialforced outwardly through the perforations to plug the thief zone. Theinformation as to the thickness of the thief zone, determined as hasbeen described, will also be useful as an indication of the amount ofcement or plugging material which will be required.

The method which has been described for injectivity profiling can alsobe used to locate a zone of lost circulation in a well. It frequentlyhappens that a very porous formation exists somewhere along the well,and these formations are often so porous that the drilling mud, whichnormally recirculates upwardly around the tubing, actually disappears inthis zone. It is, of course, necessary then to locate the zone and totreat the exposed face of the formation in some manner, as by cementing,so as to prevent this loss of the drilling fluid. It is believed to beobvious that by using the described method and forcing the interfacedownwardly while measuring its depth, a point will be found where theinterface ceases to fall even with an increased pumping rate ofradioactive fluid. This would indicate that, as fast as the radioactivefluid is pumped into the annular space between the tubing and the borehole wall, it passes outwardly into the zone of lost circulation.

It is also possible to use the method, which has been described, inselective acidization, i. e., where it is desired to inject acid into aparticular section of a formation. Thus, Fig. 7 illustrates a wellpenetrating a formation 62 which is made up of several sections ofvarying permeabilities and the lower portion of which, below the dottedline 64, is an oil section, while that portion above the line is a gassection. In such a situation, it is usually desired to force the acidinto the oil section rather than into the gas section, and the methoddescribed can be used to position an interface 32 at a point oppositethe gas-oil contact plane 64. In this case, the acid would be the liquidpumped downwardly through the tubing 18 as shown at 66, and the liquidpumped downwardly around the tubing would preferably be an oil such asis indicated at 68. As in the previous examples, a radiation detector 34would be passed through the tubing to locate the interface 32 and thepumping rates would be adjusted to maintain the interface at thisposition so that the acid 66 would enter the oil, or lower portion ofthe formation.

It is also believed obvious that the same method can be used in asituation where an oil section overlies a water section, and it isdesired to force the acid into the oil portion of the formation. In thiscase, the acid would be pumped down around the tubing and could be maderadioactive. The liquid pumped through the tubing would benon-radioactive oil or water.

Although in the description of the method which has been given it hasbeen stated that the interface 32 is moved downwardly in incrementsalong the walls of the formation 12, it is to be understood that thepermeability log of the formation can be made upwardly as well asdownwardly. Thus, the interface 32 can be positioned first at or justbelow the lower boundary of the formation 12 and then the pumping ratesof the two liquids changed by decreasing the rate of pumping at 20 andincreasing the rate at 26 by the same amount, of course, so that theinterface 32 will move upwardly to a new position along the wall of theformation. This new position or depth of the interface will then befound, as has been described, by moving the instrument 34 through thetubing and these operations may be repeated until the interface hasreached a point above the upper boundary of the formation 12. Theresulting log obtained in this manner will be the same as a log obtainedby moving the interface downwardly.

Although the radioactive liquid 24 has been described as being pumpeddown through the annular space between the tubing and the walls of thehole and the nonradioactive liquid 30 through the tubing, it is to beunderstood that the paths of these liquids can be reversed, i. e., theradioactive liquid can be pumped down through the tubing and thenon-radioactive liquid through the annulus without affecting theprocess.

While one application of this method has been described with referenceto the forcing of acid into a formation, it is to be understood that, ifdesired, almost any material can be placed in a similar manner. Forexample, a material such as a plastic or other substance which willaffect the permeability of the formation in some manner as by changingthe surface tension, plug ging pores of the formation, etc., can bepumped into a formation or portion thereof so as to increase or decreasethe permeability, if desired. Again, such a substance might be placed inthe manner described, which substance would be capable of affecting theproducing characteristics of a formation in other ways. For example, theselective placement of a suitable plastic material could be used toconsolidate the producing sands in order to prevent the production, i.e., the flowing of sand into the well with the oil.

Although the apparatus illustrated in Fig. 1 shows the use of two pumps,one to pump the radioactive liquid and the other to pump thenon-radioactive liquid, it is to be understood that a single pump can beutilized, the output stream from the pump being divided by a suitablearrangement of piping and valves and each divided stream being providedwith a suitable meter such as 22 or 28. The radioactive material ortracer can, of course, be injected into the desired one of the streams.

Obviously, many other modifications and variations of the invention ashereinbefore set forth may be made without departing from the spirit andscope thereof and only such limitations should be imposed as areindicated in the appended claims.

We claim:

1. The method of making a permeability log of a subsurface formationtraversed by a bore hole which comprises pumping a fluid into saidborehole above said formation, said fluid containing a small amount of aradioactive substance, simultaneously pumping a similar butnonradioactive fluid into said bore hole below said formation, therebyestablishing an interface between said fluids, determining the depth inthe hole of said interface by measuring the radioactivity of the fluidcontent throughout that portion of the hole being examined, varying theratio of the two fluids being pumped into the hole while maintainingconstant the sum of the two fluids so as to cause said interface to movealong the walls of said formation to another depth, again determiningthe depth of the interface by making another radiation measurement ofthe contents of the hole, and repeating these operations while notingthe ratios of the two fluids being pumped for each measured depth of theinterface in the hole.

2. The method of making a permeability log of a subsurface formationtraversed by a bore hole which comprises injecting a liquid into saidbore hole above said for tion, said liquid containing a small amount ofa radioactive substance, simultaneously injecting a similar butnon-radioactive liquid into said bore hole below said formation, therebyestablishing an interface between said liquids, determining the depth inthe hole of said interface by measuring the radioactivity of the liquidcontent throughout that portion of the hole being examined, varying theratio of the amounts of the two liquids being injected in the hole so asto position said interface substantially opposite the top of saidformation, again changing the ratio of the amounts of the two liquidsbeing injected while maintaining constant the sum of the two liquids soas to cause said interface to move along the walls of said formation toanother depth, determining again the depth of the interface by makinganother radiation measurement of the contents of the hole, and repeatingthese operations while noting the ratios of the two liquids beinginjected for each measured depth of the interface in the hole.

3. In the method described in claim in which the subsurface formation tobe examined is below the bottom of a string of casing in the hole, theadditional steps comprising: increasing the rate of injection of theliquid injected into the bore hole above said formation until theinterface moves downwardly out of the bottom of said casing, andlocating the depth of the interface below the casing while increasingfurther the rate of injection of the liquid injected above saidformation with respect to the rate of injection of the liquid below theformation, an indication that the interface remains substantially fixedjust below the bottom of the casing during said further increase in therate of injection of the liquid above said formation serving to showthat there is a leak into another formation somewhere above the bottomof the casing.

4. The method described in claim 21h which the inter face is lowered byincrements through the bore hole to determine the presence and locationof zones of lost circulation, an indication that the interface remainssubstantially stationary in the hole while the ratio of injectedradioactive liquid to injected non-radioactive liquid is increasedserving to show that all of the radioactive liquid is passing into avery permeable formation, i. e., a zone of lost circulation.

5. The method described in claim 2, in which acid is to be injected intothe oil-containing portion of a gasand oil-bearing formation and inwhich the non-radioactive liquid is an acid, the rates of injection ofthe radioactive liquid and the acid being adjusted so as to maintain theradioactive liquid-acid interface below the boundary between thegas-containing and oil-containing portions of the formation. A

6. The method described in claimj'in which the formation comprises anoil-containing portion above and in contact with a water-containingportion in which the radioactive liquid is an acid and thenon-radioactive liquid is water, the interface being maintained belowthe boundary between the oil-containing and water-containing portions ofthe formation.

7. The method of making a permeability log of a subsurface formationtraversed by a bore hole containing a tubing extending down below saidformation which comprises pumping a liquid into the annular spacebetween the tubing and the walls of the hole, said liquid containing asmall amount of radioactive substance, simultaneously pumping a similarbut non-radioactive fluid into said bore hole below said formationthrough said tubing, thereby establishing an interface between saidliquids, determining the depth in the hole of said interface bymeasuring the radioactivity of the liquid content throughout thatportion of the hole being examined, varying the ratio of the two liquidsbeing pumped into the hole while maintaining constant the sum of the twoliquids'so as to cause said interface to move along the walls of saidformation to another depth, determining again the depth of the interfaceby making another radiation measurement of the contents of the hole, andrepeating these operations While noting the ratios of the two liquidsbeing pumped for each measured depth of the interface in the hole.

8. The method as described in claim 7"in which the pumping rates areadjusted first to position the interface substantially opposite thebottom of the formation, then adjusted by increments to move theinterface upwardly along the formation walls, the position of theinterface being determined after each increment.

9. The method as described in claim in which the radioactive liquid ispumped down through the tubi while the non-radioactive liquid is pumpedinto the annular space between the tubing and the walls of the hole.

10. The method as described in claim 8 in which the radioactive liquidis pumped down through the tubing while the non-radioactive liquid ispumped into the annular space between the tubing and the walls of the ho11. The method as described in claim in which a radioactive gas ratherthan a liquid is pumped into the annular space between the tubing andthe walls of the hole while a non-radioactive gas rather than a liquidis pumped down through the tubing.

12. The method as described in claim X: in which a material capable ofaffecting the permeability of the formation is to be injected into aportion of said formation and in which one of said liquids contains saidmaterial, the rates of pumping the liquid containing said material andthe other liquid being adjusted so as to maintain the interface at aposition opposite the wall of the formation so that the liquidcontaining said material will enter said portion of the formation.

13. The method as described in claim V, in which a material capable ofaffecting the producing characteristics of a formation is to be injectedinto a desired portion of the formation and in which one of said liquidscontains said material, the pumping rates of the two liquids beingadjusted so as to maintain the interface at a position opposite the wallof the formation so that the liquid containing said material will entersaid desired portion of the formation.

14. The method as described in claim 3 in which, after it has been foundthat the liquid is leaking into another formation above the bottom ofthe casing, the position and thickness of that other formation aredetermined by measuring the radiation entering the hole from theformations above the bottom of the casing, an increase in the intensityof the radiation from any particular zone indicating that that zonecorresponds to the formation into which the liquid is leaking.

15. The method of making a log in a bore hole which comprises pumping afluid into said bore hole in a downward direction, simultaneouslypumping angther "fluid through said bore hole in an upward direction soas to establish an interface between said fluids, determining the depthin the hole of said interface, varying the ratio of the rates of the twofluids being pumped into the hole while maintaining constant the sum ofthe rates of the two fluids so as to cause said interface to move alongthe wall of said bore hole to another depth, again determining the depthof the interface, and repeating these operations while noting thedistance moved by the interface for each change in the ratio of therates of the two fluids being pumped into the bore hole.

16. The method of making a permeability log of a predetermined zone of abore hole which comprises pumping a fluid into said bore hole above saidzone, simultaneously pumping another fluid into said bore hole belowsaid zone, thereby establishing an interface between said fluids,determining the depth in the hole of said interface, varying the ratioof the rates of the two fluids being pumped into the hole whilemaintaining constant the sum of the rates of the two fluids so as tocause said inte face to move along the wall of said zone to anotherepth, again determining the depth of the interface, and repeating theseoperations while noting the distance moved by the interface for eachchange in the ratio of the rates of the two fluids being pumped into thebore hole.

17. The method as described in claim 16 in which one of said fluidscontains a tracer material.

Wolf Sept. 28, 1948 Arthur Aug. 4, 1953

